Gas, oil, and water coexist in the stratum of an oil/gas production well. Since gas and oil have specific gravities lower than that of water, they are floating on the water. When the oil/gas production reaches a certain volume, formation water would gradually rise to approach perforated zones in the well and tends to enter the production well.
Generally, entering of water into a production well might be caused by different reasons, such as a high permeability formation at the production zone that results in an earlier edge encroachment on the well, vertical cracks or faults within the production zone that result in flowing of bottom formation water into the upper production zone, fissures on a casing or a plug set in the well, and poor cement bond at the casing of the well. Or, a rising water gas contact (WGC) would induce water coning to resulting in increased water rate and shutoff of well. Under these conditions, the water must be completely shut off lest it should rise and enter the well to adversely affect the production at other zones.
In the case of a gravel packed production well subjected to water encroachment, since the gravels between a screen pipe and a wall of the well is highly permeable and there is not a casing provided at the production zone, both of the mechanical type bridge plug and the thermal expansion type patch flex are not applicable to block the perforated zone under the WGC for the purpose of continuing the oil/gas production of the well.
The conventional bridge plug can only be mounted in the screen pipe, and does not provide the function of blocking tiny meshes on the screen pipe and the gravels outside the screen pipe. That is, the conventional bridge plug does not function to seize invasive water from flowing into the well. Similarly, the thermal expansion type patch flex must also be provided in the screen pipe. As to the conventional way of pumping gel into the gravel packed production well using coiled tubing, since the tiny meshes on the screen pipe and the gravels outside the screen pipe are highly permeable, the pumped gel would inevitably flow through the gravels and fails to completely enter a water out zone in the production well.
Regarding the way of placing traditional G-grade or H-grade cement milk at the well bottom to block the water out zone, it is also not applicable because the cement has large grain size and does not pass through the screen pipe into the gravels easily. Therefore, once the bottom of the gravel packed production well is subjected to invasion by water, there is no way to save the well but leaving it to the rising water until the well is no longer economical for use and shut off, or until the invaded water accumulates in the oil pipe and the well no longer produces any oil or gas.
For gravel packed production wells, a most common way of stopping water invasion is to use thermosetting resin or fine-grained cement milk to block the water out zone in the production well. In most cases, the resin used for this purpose is phenolic resin. However, in the case of stopping water invasion with the thermosetting resin, the formation water would still break through a joint of the gravels and the well wall to largely reduce the water blocking effect when a differential pressure at the well bottom exceeds 50 psi. There is also limitation to the cement used to block water. Only the fine-grained cement can flow into the gravels to completely fill up the pore space thereof, and thus blocks the water out zone after the cement is cured.
In performing water blocking for the gravel packed production well, a dump bailer is used to carry the resin or cement. The dump bailer is a cylindrical container being lowered to the well bottom or a desired depth in the well using a wire line. A signal is transmitted from the ground to open the dump bailer and release the resin or cement milk loaded therein. Due to a difference in specific gravity between the resin or cement milk and the formation water, the resin or cement milk is able to flow into the meshes and the gravels. When the resin or cement milk is cured, the water out zone is blocked.
However, in the conventional way of releasing the cement from the dump bailer, it is not able to control a flow rate of the released cement. Once a bottom opening of the dump bailer is opened, the cement in the dump bailer immediately flows out to mix with the formation water, resulting in an excessively low concentration of the cement and preventing the cement from curing effectively to thoroughly block the water out zone. That is, the quickly released cement does not provide good water blocking effect and the formation water tends to rise via the incompletely filled pore spaces of the gravels to adversely affect the oil/gas production at other production zones.
When the dump bailer is operated while the well has water therein, the releasing speed of the cement milk has great influence on the mixture of the cement milk with water, and accordingly the concentration of the cement milk. When the cement milk is released at a high speed, the cement milk mixed with the formation water is highly diluted to the extent of being unable to cure and losing the function of blocking water. This condition can be observed in lab experiments. On the other hand, when the cement milk is released at a controlled slow speed, the undesirable dilution of the cement milk can be largely improved.